I have extremely mixed feelings about it, myself.
Done right, it's seamless, you can't tell it ever happened, and it works really, really well, and means it costs you
far less time and effort. 4E allowed the re-skinning of monsters in such a way (via the DDI particularly) that it was virtually impossible, short of forensic analysis, to determine that it had happened.
Done poorly, or with bad choice of material, it's dreadful, and can be deeply anti-immersive, and think can lead to some really unhelpful suggestions on how to approach/deal with certain issues.
Most often I see it done will with re-skinning monsters. This tends to be highly successful as they are subject to limited scrutiny and the players frequently will never even know the mechanics they use, merely the results. I see it done poorly most often with classes, particularly where a class has extremely strong or distinctive mechanics. People can be prone to saying, rather dismissively, "just reskin X class as Y idea!", and yeah sometimes that works, but sometimes it really doesn't. Not that long ago I saw someone try to continue a 4E character into 5E, and the closest he could get was a heavily re-skinned Warlock (also re-skinning a race as another race). I suggested he should probably make a new character, or re-work the concept on a more basic level, but he went with it, and he clearly didn't enjoy it, and it kept sort of breaking down, the illusion faltering, as it were.
I'm kind of surprised but glad to see a lot of people not the potential issues with it!
